This application claims the priority to Japanese Patent Application No. 2000-357859, filed on Nov. 24, 2000.
The present invention relates to a golf club shaft made of fiber reinforced plastics (or an FRP golf club shaft), and in particular it relates to a FRP golf club shaft which facilitates a swing by increasing modulus of longitudinal elasticity with increases in head speed during the swing, and a golf club having such a shaft.
A club shaft made of fiber reinforced plastics (hereinafter referred to as an xe2x80x98FRP golf club shaftxe2x80x99) is advantageous over metal golf shafts in that it has lighter weight than metal ones, which makes it easier to accelerate a swing and increase the flying distance. Thus, the FRP golf club shaft is extensively employed.
The FRP golf club shaft is a shaft formed of reinforcing fibers impregnated with resin. Types of FRP shafts include a shaft fabricated in the sheet rolling process (S/R shaft), a shaft fabricated in the filament winding process (FW shaft), and a braided shaft. The S/R shaft is formed by winding unidirectional prepreg sheets made of reinforcing fibers over a mandrel. The FW shaft is formed by winding a fiber bundle (yarn) of reinforcing fibers around the mandrel while reciprocating them along the longitudinal axis of the mandrel. The braided shaft is formed by braiding a plurality of fiber bundles (yarn) of reinforcing fibers or tow prepregs (or yarn prepregs) while braiding them over the mandrel to the substantially entire length of the shaft. The braided shaft is superior in improved bending strength since no joint exists both in the longitudinal and circumferential directions of the shaft and braid yarns are intertwined.
For example, JP-A-6-278216 discloses a braided shaft. The braided shaft is formed by intertwining a plurality of diagonal yarns, which are positioned symmetrically at a certain orientation against the longitudinal shaft axis, and warps positioned at 0xc2x0 against the longitudinal shaft axis. The diagonal yarns and warps are interwoven to form a triaxial braid layer, which improves the bending strength of the shaft.
JP-A-11-342233 also discloses a braided shaft that is formed by laminating a plurality of braid layers. When all the braid layers have the triaxial construction that has a plurality of diagonal yarns set symmetrically against the longitudinal axis and warps set at 0xc2x0 against the longitudinal axis, the three types of yarn overlap with each other at some portions and do not overlap at the others. In other words, the height differs at these portions. Thus, the shear strength between the braid layers decreases, and the bending strength and torsional strength are weakened. Consequently, conventional braided shafts have triaxial construction in its outer braid layer and biaxial construction in its inner braid layers.
The conventional shafts address an improvement in bending strength by including triaxial braid layers having symmetrical diagonal yarns and warps. However, during a swing, centrifugal load on the shaft increases with increases in the head speed, thus affecting the shaft performance (i.e., bending deformation of the shaft and the corresponding change in feelings or flying distance). Therefore, the relationship between the load due to centrifugal force and deformation should be studied more in detail.
Consider the state of the shaft during a swing. The golfer causes generally rotational motion of the head of the club during a swing to hit a ball. It is assumed that (1) centrifugal force immediately before the impact and (2) inertial force caused by acceleration or deceleration of the head are applied to the shaft. More specifically, the centrifugal force of (1) is 300 to 500 N which is generated immediately before the impact, during which time the head speed reaches 40 to 50 m/s. This force pulls the entire shaft in the centrifugal direction of the rotational motion and causes bending deformation and tensile deformation in the shaft. The inertial force of (2) originates in acceleration or deceleration of the head when the golfer rotates, twists, or translates his waist, arms, or wrists. This force applies bending or torsional moment on the shaft, thus causing its bending or torsional moment. The centrifugal and inertial forces produce (A) tensile stress and compression stress symmetrical to the neutral plane that are caused by bending moment on the shaft that is applied in the direction of the shaft and (B) tensile stress in the longitudinal direction that is caused by centrifugal force.
The neutral plane of (A) means a virtual plane located along the longitudinal axis of the shaft upon which no tensile stress and compression stress act. The centrifugal force particularly acts on the shaft with significant force in the state before the impact, in which the club is swung down and the head speed has increased to some extent. In this state, the tensile deformation in the centrifugal direction and compression in the circumferential direction of (B) are negligibly small, while the bending moment of (A) increases with the acceleration of the head speed. In other words, as the centrifugal force on the shaft increases, the shaft deflects in a complicated fashion in combination with the change in the inertial force caused by the swing. Great increases in the bending moment change the deflection of the shaft greatly, which makes the degree of deformation unstable in the conventional shafts. This sometimes affects swings or feels (such as stability) perceived by golfers when they swing the golf clubs. Professional golfers, who have relatively high physical power and thus cause fast head speed and great deformation of the shaft or who have a more sensitive touch, are more likely to notice this shaft deformation. In particular, the shaft deformation immediately before the impact affects the trajectory of the balls hit. Therefore, it is important to suppress the shaft deformation and to stabilize the hit of the balls.
As shown in FIG. 10, in the shaft having a braid layer with the conventional triaxial construction, diagonal yarns 41 and 42 are symmetrically positioned at the degrees of orientation of +xcex8 and xe2x88x92xcex8 respectively against the warps 40 provided parallel to the longitudinal axis 43 of the shaft, and every other intersection of the diagonal yarns 41 and 42 exists on the warps 40. When the average width of the braid yarns 40, 41, and 42 is designated as t (mm) and the length perpendicular to the warps 40 at the intersection of the diagonal yarns 41 and 42 (or the length in the circumferential direction) is designated as "igr" (mm), the braid yarns are alternately positioned in the order t, "igr", t, "igr", and so forth. Since "igr"=t/cosxcex8, the equation t+"igr"=t+t/cosxcex8 is obtained. When the numbers of yarns are respectively set as n for the warps 40 and the diagonal yarns 41 and 42, sets of t+"igr" lined n times correspond to the entire circumference of the shaft. When the diameter of the shaft 2 is designated as D (mm), the circumference may be expressed as follows;
xcfx80Dxe2x88x92n(t+t/cos xcex8) 
In the conventional braid layers, since overlaps at a single point of three types of yarns 40, 41, 42 repeatedly exist, the height at the triaxial overlaps differs from that at other portions without the triaxial overlaps, thus increasing three-dimensional spaces S between the yarns. When the load on the shaft due to centrifugal force increases with acceleration of the head speed during a swing, the spaces S get smaller plastically, which increase deformation of the shaft.
The object of the present invention is to provide a golf club shaft made of fiber reinforced plastic, which facilitates swing by suppressing deformation of the shaft by the centrifugal force applied to the shaft before the impact.
A golf club shaft made of fiber reinforced plastics has a braid layer along the length of the shaft that includes first diagonal yarns and second diagonal yarns. The first diagonal yarns have a first orientation angle against the longitudinal axis of the shaft. The second diagonal yarns have a second orientation angle, which is symmetrical with the first orientation angle, against the longitudinal axis of the shaft as the center axis. The ratio of the longitudinal modulus of the shaft during a swing to the longitudinal modulus of the shaft when the head speed is zero increases with the increase in the head speed.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.